Metal halide lamps began with the addition of the halides of various light-emitting metals to the high pressure mercury lamp in order to modify its color and raise its operating efficacy as proposed by U.S. Pat. No. 3,234,421--Reiling, issued in 1966. Since then metal halide lamps have become commercially useful for general illumination; their construction and mode of operation are described in IES Lighting Handbook, 5th Edition, 1972, published by the Illuminating Engineering Society, pages 8-34.
The metal halide lamp generally operates with a substantially fully vaporized charge of mercury and an unvaporized excess consisting mostly of metal iodides in liquid form. The favored filling comprises the iodides of sodium, scandium and thorium. The operating conditions together with the geometrical design of the lamp envelope must provide sufficiently high temperatures, particularly in the ends, to vaporize a substantial quantity of the iodides, especially of the NaI. In general, this requires minimum temperatures under operating conditions of the order of 700.degree. C.
The quantity of NaI which may be accommodated in the vapor state within a given volume at a given temperature, for instance at 750.degree. C., can be readily calculated. However, the charge of NaI that is put into most lamps of commercial manufacture is many times greater, for instance 100 or more times, than such calculated quantity. Although most of the added NaI remains as condensate within the arc tube, the quantity participating in the arc discharge increases at a diminishing rate with the total quantity put into the tube. In Electric Discharge Lamps, MIT Press 1971, Chapter 8, Section 8.4, Effects of Arc Tube Geometry, John F. Waymouth speculates on this phenomenon and proposes, as explanation based on the non-isothermal nature of the bulb, that a film of condensate spreading beyond the point of minimum temperature towards higher temperatures would result in increasing the NaI pressure. He also offers an alternative explanation wherein the NaI pressure in the arc is viewed as being determined by a dynamic rather than an equilibratory process; convection currents bring gases that are much hotter than the wall past the surface of the condensate film, evaporating excess NaI to be carried through the arc before condensing elsewhere.
Irrespective of the explanation adopted, Waymouth concludes that it is desirable to have a condensate film as extensive as possible to get the maximum pressure of NaI in the gas for a given quantity of NaI added. In particular, he desires that the condensate be distributed over the barrel of the arc tube, spread as thinly as possible over as large an area as possible, and not condensed in the ends where there might be crevices or pockets that could store relatively large quantities with low surface area. To achieve this result, Waymouth wants the arc tube designed in such a way that the end temperatures are higher than those in the middle, so that excess iodide will condense in the middle of the arc tube.
We have observed that the condensate does not form a true film in the sense of a continuous layer on the inside of the quartz arc tube, but tends to remain as discrete droplets. We have found that the extent to which the area of the condensate can be increased by observing the Waymouth recommendations is quite limited. Also we have encountered other problems when much condensate coats the envelope walls about the middle of the arc tube, for instance as an equatorial band spaced away from both electrodes. The relatively large droplets of condensate in the band reduce transmission and may cause flickering as they form and move about. Another problem is the occurrence of flashes of reddish light, particularly during warm-up. These flashes appear to be due to rapid vaporization of drops of metal halide dose which form in the equatorial band and run down into the hotter end zone. Such problems are particularly acute in miniature metal halide arc tubes of one cubic centimeter or less such as disclosed in U.S. Pat. No. 4,161,672--Cap et al, July 1979.